When exposed to high temperatures (i.e., greater than or equal to about 1,300° C.) and to oxidative environments, metals can oxidize, corrode, and become brittle. These environments are produced in turbines used for power generation applications. Thermal barrier coatings (TBC), when applied to metal turbine components, can reduce the effects that high-temperature, oxidative environments have on the metal components.
Thermal barrier coatings can comprise a metallic bond coating and a ceramic coating. The metal bond coating can comprise oxidation protection materials such as aluminum, chromium, aluminum alloys, and chromium alloys. For example, the metallic bond coating can comprise chromium, aluminum, yttrium, or combinations of the forgoing, such as MCrAlY where M is nickel, cobalt, or iron (U.S. Pat. No. 4,034,142 to Hecht, and U.S. Pat. No. 4,585,481 to Gupta et al. describe some coating materials). These metallic bond coatings can be applied by thermal spraying techniques (Gupta et al. describe the coating materials comprising silicon and hafnium particles being applied by plasma spraying). The ceramic coating can be applied to the metal bond coating by methods such as air plasma spray (APS) or electron beam physical vapor deposition (EB-PVD).
U.S. Pat. No. 6,042,898 to Burns et al., teaches applying a thermal barrier coating by depositing a MCrAlY bond coat onto a superalloy substrate. Burns et al. teach forming an aluminum oxide scale on a MCrAlY bond coat and depositing a ceramic layer on the aluminum oxide scale using physical vapor deposition. Burns et al. teach enhanced coating life using an ionized gas cleaning process, such as reverse transfer arc cleaning. This process entails forming an arc that superheats oxides and other contaminants on the blade's surface, causing the oxides and contaminants to vaporize. The process is performed at pressures of 30 torr absolute (4.0 kPa) to 40 torr absolute (5.3 kPa) and temperatures of 1,400° F. (760° C.) to 1,600° F. (871° C.).
When the ceramic coatings are applied to the metallic bond coating comprising aluminized MCrAlY and/or over dense high velocity oxy-fuel flame (HVOF) coatings, the ceramic coating can exhibit poor adhesion. HVOF is a supersonic process, which can deliver gas velocities at over 6,000 feet per second (fps), that allows particle velocities of over 3,000 fps and that can produce coatings with high bond strengths. It is an extremely versatile system that offers an unlimited range of possibilities to industries with extreme corrosion and wear environments. However, the resultant coatings are smooth and enable limited adhesion with subsequent coatings. Hence, there exists a need for an improved method to adhere a ceramic coating to these smooth coatings.